OUTREACH BRIEF: TIPS FOR MANAGING SOUR ROT IN THE WINERY - BELINDA KEMP, EMMA SNYDER, WENDY MCFADDEN-SMITH, AND DEBBIE INGLIS

OUTREACH BRIEF:
TIPS FOR MANAGING
SOUR ROT IN THE WINERY
Belinda Kemp, Emma Snyder,
Wendy McFadden-Smith,
and Debbie Inglis

Outreach Brief: Tips For Managing
                                            Sour Rot in the Winery

Introduction
Sour rot is a recurring problem in early season thin-skinned, tight-clustered varieties like
Pinot noir, Pinot gris, Riesling, Gewurztraminer, Gamay and Baco noir. Sour rot results from
interactions between multiple yeast species naturally present on grapes and acetic acid
bacteria i.e. Gluconobacter and Acetobacter spp (Barata et. al. 2012). The causal organisms gain
entry to the berries via microfissures around the pedicel, berry splitting, or due to other berry
injuries. Berries become susceptible to infection between 10 and 15° Brix. Spread of the disease
is linked to Drosophila spp. It is hypothesized that they carry the yeasts and bacteria from
infected to healthy berries and/or that fly larvae contribute to the dessication of the berry (Hall
et. al. 2018). Sour rot has been responsible for more than $1.5 million in crop loss in a given
season in the Ontario region, when weather conditions promote infection, such as rain storms
and hail in 2008 (Agrocorp, 2008).

Compounds associated with sour rot in juice and wine include acetic acid, ethyl acetate,
acetaldehyde, galacturonic acid, gluconic acid, phenylacetic acid (PhAA), and ethyl
phenylacetate (EPhA) (Campo et al. 2012). Acetic acid and ethyl acetate are the primary
contributors to volatile acidity (VA), a common wine fault. The maximum permitted VA
concentration in still table wine in Ontario is 1.3 g/L (Vintners Quality Alliance 1999). However,
the consumer rejection threshold of acetic acid (vinegar aroma) and ethyl acetate (acetone/
varnish aroma) are 0.8 g/L and 7.5 - 200 mg/L respectively. Ethyl phenylacetate (EPhA) and
phenylacetic acid (PhAA), when present together in wine, have been found to bestow an
undesirable “sweet/honey” off-flavour at high levels, which is considered a fault (Campo et al.
2012). The consumer rejection threshold for a combination of both compounds is over 140 ug/L
of ethyl phenylacetate (EPhA) and at 700 ug/L or higher for phenylacetic acid (PhAA) (Campo
et al. 2012, Kemp et al. 2019, 2020).

In years with high sour rot production, it may not always be possible to pick grapes earlier than
anticipated or sort bunches. In this case, winemaking strategies to deal with sour rot in the
winery are required, according to wine style and grape variety. These winemaking guidelines for
sour rotten grapes have been compiled to help winemakers minimise the negative impacts of
sour rot on wine quality and decrease a loss of income from reduced yield.

Outreach Brief: Tips For Managing
                                             Sour Rot in the Winery

Harvest
Fruit should be sorted in the vineyard prior to harvest or at picking, by staff trained to identify
sour rotten grapes. If infection is high, sulfur dioxide (SO2) can be added to the picking bins at
a rate of 30 - 50 ppm as potassium metabisulphite (KMS). For those who wish to reduce the
use of SO2, there are several bioprotectant yeast preparations currently on the market that aim
to reduce VA production by outcompeting sour rot causing microorganisms. Protection, either
via the use of SO2 or bioprotection, is important in a warm harvest when fruit has to wait in
the vineyard for collection or travel long distances to a winery for processing. If fruit is machine
harvested, grapes should be sorted outside the winery, either manually, on a conveyor belt,
or using an optical sorter. Ensure the picking bins, winery equipment, and tanks are sterilised
before and after use to prevent contamination by undesirable microbes.

Juice processing options
Sparkling and white grape juice:
Whole bunch press as normal, but at a lower pressure than when processing healthy fruit.
Add SO2 to the juice tray after pressing at 50 - 80 ppm. As with healthy fruit, fractionate the
juice and vinify the fractions separately. The use of settling enzymes at a high rate and cold
temperatures during settling is recommended to reduce microbial activity.

Red juice:
Destemming and crushing of red grapes should be gentle, and SO2 can be added to the must
at 20 - 50 ppm (or higher depending on the choice of yeast), and depending on the severity
of the infection. It is preferable to avoid pre-fermentation maceration/cold soak, and instead
inoculate with a robust, rapid yeast for alcoholic fermentation. If needed, acid addition should
be considered to lower pH levels of the juice, especially for low colour/tannin varieties that are
susceptible to colour loss.

Outreach Brief: Tips For Managing
                                            Sour Rot in the Winery

Juice fining
Juice fining agents that can be used prior to fermentation include bentonite to remove
rot-derived proteins, polyvinylpolypyrrolidone (PVPP) and/or activated carbon to reduce the
incidence of browning. Targeted addition of fining agents, via bench trials, is recommended as
over use is associated with the removal of other compounds.

Nutrition
Yeast Assimilable Nitrogen (YAN mg N/L) must be analysed before the yeast inoculation to
ensure low or high additions are avoided. YAN should be added according to the nutrient
demands of the chosen yeast; low YAN levels can lead to sluggish or stuck fermentations,
while high YAN can allow undesired yeast and bacteria to thrive. Yeast nutrient that contains a
mixture of DAP and organic forms of nitrogen are advisable. DAP, which is primarily composed
of Ammonium (NH4+) has been reportedly associated with higher concentrations of ethyl
acetate in synthetic grape must fermentations (Beltran 2005).

Yeast and fermentation
The primary fermentation of sparkling wines should be carried out at low temperature to
reduce bacterial and enzymatic activity. The yeast strain chosen needs to be a sparkling wine
yeast that consumes acetic acid (Vasserot et al. 2010), is tolerant of high SO2 and high volatile
acidity, and a strong fermenter with low nutrient requirements. Ensure oxygen is limited in
both sparkling primary fermentation to reduce the chance of microbial spoilage. For red wines,
a fast fermentation at high temperatures is recommended. High temperature fermentation
(30°C) can reduce acetic acid levels. It will also help the inoculated yeast and prevent
competition from non-saccharomyces yeast and bacteria on grape skins. Where malolactic
fermentation (MLF) is desired, co-inoculation with a compatible O. oeni strain is recommended
to facilitate the rapid completion of MLF soon after the end of alcoholic fermentation and
reduce the potential of bacterial spoilage (Abrahamse and Bartowsky 2012; Tristezza et. al.
2016).

Outreach Brief: Tips For Managing
                                            Sour Rot in the Winery

Fermentations must be monitored twice a day (morning and evening), and exposure to oxygen
during cap management should be minimized; for example, by replacing pumpovers with
punchdowns/plunging.

Techniques that add colour and promote colour stability are recommended as time on skins
should be limited. Enzymes that assist in extracting phenolic compounds from grapes (pigment
and tannin), especially for low tannin varieties like Gamay and Pinot noir, are recommended
for enhanced colour extraction and stability (Marcotte et al. 2021). Other treatments have
been reported in the literature to stabilize and increase colour in red wines. These include the
use of untoasted oak dust, yeast hulls, activated tannins, increased SO2, grape seed tannin, and
neutral oak chips. Post-fermentation maceration is not recommended.

Sulfur dioxide (SO2) additions post-fermentation
Monitor pH levels and ensure SO2 additions post-fermentation are made according to pH level.
The risk of high SO2 addition to low colour/tannin wines (i.e. Gamay and Pinot noir) can further
bleach the colour, hence colour and tannin stabilisation during fermentation is essential so that
the pigment gets bound by tannin and traps it in a coloured form making it resistant to further
bleaching by SO2 . Precautions that should be taken to prevent oxidation include: the use of dry
ice, sparging with nitrogen or carbon dioxide (CO2) gas, minimizing ullage in tanks and storage
vessels, and reducing the incidents of moving/racking wines.

Fining and filtering sparkling base wine & red wine
Reduce time on primary lees by racking off solids as soon as possible after primary
fermentation. Your local supplier will have details of specific fining agents that help to break
down large compounds to aid in filterability. Filtration of all wines produced from sour rotten
grapes prior to bottling at 0.45 μm is recommended.

Outreach Brief: Tips For Managing
                                           Sour Rot in the Winery

Alternative solutions
In extreme cases, reverse osmosis (RO) can reduce the concentration of VA; however, RO may
also remove desirable compounds in addition to VA. Mobile RO units may be available in your
region or another winery may offer that service. Winemakers can also conduct trials to test the
potential of the wines made from sour rotten grapes in a blend, although ensuring the
microbial stability of the wine before addition to a blend is of primary concern.

CCOVI Analytical Services
CCOVI Analytical Services provides a wide range of analyses to Ontario wineries for juice and
wine analysis from ripening through to finished wine. These include sour rot-associated
compounds (acetic acid (g/L), ethyl acetate (mg/L). Please contact ccovilab@brocku.ca for a
full list of services.

Acknowledgements
The authors thank the Canadian Grapevine Network Certification (CGCN) and Ontario Grape
and Wine Research Inc (OGWRI) for their funding and support for this project. Additionally, we
thank Craig McDonald, Vice President - Winemaking at Andrew Peller Limited, and Emma
Garner, Winemaker, Trius Winery, for their collaboration on this project and contribution
towards these guidelines. Sour rot photograph courtesy of Dr. Wendy McFadden-Smith,
OMAFRA.

Outreach Brief: Tips For Managing
                                              Sour Rot in the Winery

References
Abrahamse, C. E., & Bartowsky, E. J. (2012). Timing of malolactic fermentation inoculation in
Shiraz grape must and wine: influence on chemical composition. World Journal of Microbiology
and Biotechnology. 28(1), 255-265.
Barata, A., Pais, A., Malfeito-Ferreira, M., & Loureiro, V. (2011). Influence of sour rotten grapes
on the chemical composition and quality of grape must and wine. European Food Research and
Technology. 233, 183–194.
Barata, A., Malfeito-Ferreira, M., & Loureiro, V. (2012). Changes in sour rotten grape berry
microbiota during ripening and wine fermentation. International Journal of Food Microbiology,
154(3), 152-161.
Beltran, G., Esteve-Zarzoso, B., Rozès, N., Mas, A., & Guillamón, J. M. (2005). Influence of the
timing of nitrogen additions during synthetic grape must fermentations on fermentation
kinetics and nitrogen consumption. Journal of Agricultural and Food Chemistry, 53(4), 996-
1002.
Campo, E., Saenz-Navajas, Cacho, J., & Ferreira, V. (2012). Consumer rejection threshold of
ethyl phenylacetate and phenylacetic acid, compounds responsible for the sweet-like off odour
in wines made from sour rotten grapes. Australian Journal of Grape & Wine Research. 18,
280–286.
Hall, M. E., Loeb, G. M., Cadle-Davidson, L., Evans, K. J., & Wilcox, W. F. (2018). Grape sour rot: a
four-way interaction involving the host, yeast, acetic acid bacteria, and insects.
Phytopathology, 108(12), 1429-1442.
Kemp, B., Pickering, H., Small-Kelly, S., Blake, A., Bowen, A., Inglis, D and Pickering, G. (2019).
Improving sparkling and still red wine quality: Honey, sweet off-flavour from sour rot. British
Columbia Wine Growers Council (BCWGC) Oenology and Viticulture Conference. Penticton
Trade & Convention Centre, Penticton, BC. Canada. 15-16th July 2019.

Outreach Brief: Tips For Managing
                                            Sour Rot in the Winery

References
Kemp, B. (2020). Effect on honey, dusty off-flavours and acetic acid in sparkling wines made
from varying amounts of sour rotten grapes. CCOVI Lecture Series. 6th April 2020.
https://www.youtube.com/watch?v=PLSVsKKCK90
Marcotte, C, Kemp, B. and Inglis, D. (2021). Enzyme treatments positively impact tannin
extraction in Pinot noir and Cabernet sauvignon red wine from Niagara. Ontario Fruit and
Vegetable Convention (OFVC). 5th March 2021. https://www.ofvc.ca/posters.html
Tristezza, M., di Feo, L., Tufariello, M., Grieco, F., Capozzi, V., Spano, G., & Mita, G. (2016).
Simultaneous inoculation of yeasts and lactic acid bacteria: Effects on fermentation dynamics
and chemical composition of Negroamaro wine. LWT-Food Science and Technology, 66, 406-
412.
Vasserot, Y., Mornet, F., & Jeandet, P. (2010). Acetic acid removal by Saccharomyces cerevisiae
during fermentation in oenological conditions. Metabolic consequences. Food Chemistry, 119,
1220–1223.